1. Field of the Invention
The present invention relates to a method of controlling a receiving operation which is used for wireless LAN(Local Area Network).
2. Description of the Related Art
Wireless LAN terminals are increasingly a part of modern life. The wireless LAN terminals do not need a network cable. Therefore, in the wireless LAN terminal which is driven by a battery, the terminal can be used as a mobile device in a communication area. For making a communication between the wireless LAN terminals and an accessing point, CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance) is used as a wireless LAN standard. The CSMA/CA is used by a 2.4-GHz band wireless LAN standard such as an IEEE(Institute of Electrical and Electrical Engineers) 802.11b standard and an IEEE 802.11g standard.
A maximum transmission speed of the IEEE 802.11g is 54-Mbps and a maximum transmission speed of the IEEE 802.11b is 11-Mbps. The IEEE 802.11g has an upward compatibility to the IEEE 802.11b. Therefore, a device which uses the IEEE 802.11g can communicate to a device which uses the IEEE 802.11g and the IEEE 802.11b. Both of the IEEE 802.11b and the IEEE 802.11g adopt DSSS(Direct Sequence Spread Spectrum) and OFDM(orthogonal frequency division multiplexing). A synchronizing portion and a header portion of a transmission frame are modulated by a DBPSK of 1 Mbps mode or a QPSK of 2 Mbps mode for making the compatibility of these standards. The IEEE 802.11b and an IEEE 802.11a are described in reference 1: Japanese Patent Laid-Open No. 2003-299141.
In the mobile terminal of the wireless LAN system, low power consumption is desired. A technique for achieving the low power consumption is described in reference 2: Japanese Patent Laid-Open 8-195754. In reference 2, a standby period is extended by receiving a beacon signal for reducing the power consumption during waiting time. The mobile terminal may be called a mobile station or simply called a wireless LAN station in this invention.
The beacon signal is transmitted from the access point every 100 msec. Therefore, the mobile station which has a power saving mode can be changed between an awake mode and a doze mode in response to the beacon signal from an access point. The access point changes the condition of the mobile station to the awake mode or the doze mode by checking whether the mobile station is in a power consumption mode or not. An information which indicates an interval of the transmitting operation of the beacon signal is included in a frame body which is next to a MAC header portion.
In the wireless LAN station, further low power consumption has been desired. In particular, the device which is driven by a battery desires the low power consumption. In the CSMA/CA, the mobile station receives carriers continually for checking a data sending timing. Therefore, it is difficult to reduce the power consumption in such device.
For example, when the access point and one mobile station communicate using the IEEE 802.11b, and the other mobile station which uses the IEEE 802.11b and a mobile station which uses the IEEE 802.11g are located in the same communication area, a radio wave of the IEEE 802.11b is transferred to each station in the communication area. The station of the IEEE 802.11g waiting to send a signal may wait until the communication which is performed by the IEEE 802.11b is completed. Therefore, the station which uses the IEEE 802.11g and is waiting to send the signal, consumes powers receiving the unneeded signal.
Also, when a plurality of mobile stations which use the IEEE 801.11g make a communication link to the access point, the stations waiting to send the signal may wait while receiving the unneeded signal until the communication by the other station is completed. Therefore, it is difficult to reduce power consumption.